Conventional x-ray radiography records the attenuation of x-ray radiation, over the surface of an image plane, after it has passed through a patient. The attenuation is typically recorded as a pattern or "image" on a sheet of x-ray film.
The pattern of attenuation ideally indicates the relative opacity (to x-rays) of the patient along many rectilinear "rays" extending from the x-ray source through the patient to the film. Ideally, each point of the film image indicates the total attenuation caused by internal structures of the patient along a single ray.
In practice, however, when x-ray radiation passes through a patient, a certain amount of the radiation is scattered away from its path of incidence. Some of this scattered radiation is still received by the film, although at a point other than where it was originally directed. The scattered radiation causes portions of the x-ray image to receive additional x-ray energy that may not have been attenuated by structure of the patient directly interposed between that portion of the image and the x-ray source. The amount of scatter depends on the material through which the x-rays pass. For example, less scatter is encountered in imaging the lungs, which are of low density, as compared to the mediastinum which is a relatively higher density.
The net effect of scatter is that the contrast of the image, the difference between light and dark portions of the image, is degraded. The contrast of an image, all other things being equal, effects the amount of information conveyed by the image. A decrease in contrast may result in the loss of diagnostically important information.
Scatter has heightened significance in certain applications, such as dual energy bone densitometry, where the attenuation at each portion of image at two energies is determined quantitatively and mathematically combined to isolate different tissue within the patient. Here small amounts of scatter that might be tolerable on a qualitative basis can cause unacceptable quantitative errors.
It has long been known that scatter in conventional radiography may be controlled by the use of a grid consisting of a series of regularly spaced thin plates or lamellae arranged edgewise to allow passage of x-rays only along a straight line path from the x-ray source to the image receptor. Scattered x-rays that do not travel along a straight line path see a much greater area of lead and are preferentially absorbed.
The effectiveness of a grid in passing desirable or "primary" x-rays is measured by its "primary transmission" and depends generally on the ratio of the lamellae's thickness to the space between the lamellae, i.e., the "intergrid spacing" and the "lead content" (mg/cm.sup.2) of the grid. Thinner lamellae and greater spacing between the lamellae block fewer primary x-rays. A typical grid may have a primary transmission of approximately 70% and thus there is a significant reduction in total exposure of the film caused simply by the use of the grid. A decrease in exposure of the film, like a decrease in contrast, can reduce the amount of information contained in the image and cause the loss of diagnostically significant details in the image. Accordingly, the use of a grid is not without cost in terms of diagnostic information and the use of a grid is typically considered only when its effect on the reduction of scatter is expected to be significant.
The effectiveness of the grid in blocking oblique or scattered x-rays depends generally on the height of the lamellae, as measured along the rays, in proportion to the spacing between the lamellae. Higher lamellae and lamellae that are spaced closer together block more scattered radiation. The height of the lamellae in proportion to their spacing is typically expressed as a "grid ratio". Typical grid ratios are 8:1 and 12:1 meaning that the lamellae are respectively eight or twelve times as high as the spacing between them.
Grids having strip densities (lines/mm) of substantially less than 100 lines per inch can often produce objectionable grid lines on the resulting image, the grid lines being the shadows of the lamellae. Moving the grid during the x-ray exposure of the image receptor blurs the grid lines over a larger area thus rendering them fainter and thus less objectionable.
With the advent of scanning radiography, where the area x-ray beam is replaced with a highly collimated pencil or fan beam, the problems of scatter have been remarkably reduced. In such systems, the collimated radiation beam is moved in a scanning pattern over an area of the patient to be imaged. Synchronously, a collimating slot is moved to remain opposed to the radiation beam on the opposite side of the patient. Only a portion of the image is exposed at any given time.
The effect of the highly collimated radiation beam and the slot is to eliminate the effect of scattered radiation from rays normally present on either side of the collimated beam during the exposure of any given portion of the image. With suitably narrow radiation beams, the problem of scatter from adjacent rays is virtually eliminated.
Narrowly collimated radiation beams may create significant tube loading problems. Specifically, in order to provide an acceptably short scanning time the radiation beam must provide no less than a certain minimum fluence. The fluence is generally proportional to both the area of the collimated beam and the power of the x-ray tube. Collimation of the x-ray beam to increasing small areas requires correspondingly greater x-ray tube power and much of that increased power is wasted by the narrow collimation. Thus, in practice, extremely narrow radiation beams may be inefficient or unduly expensive.